Magnetically regulated gas accretion in high-redshift galactic disks

Abstract

Disk galaxies are in hydrostatic equilibrium along their vertical axis. The pressure allowing for this configuration consists of thermal, turbulent, magnetic, and cosmic-ray components. For the Milky Way the thermal pressure contributes 10% of the total pressure near the plane, with this fraction dropping toward higher altitudes. Out of the rest, magnetic fields contribute ∼1/3 of the pressure to distances of ∼3 kpc above the disk plane. In this Letter, we attempt to extrapolate these local values to high-redshift, rapidly accreting, rapidly star-forming disk galaxies and study the effect of the extra pressure sources on the accretion of gas onto the galaxies. In particular, magnetic field tension may convert a smooth cold-flow accretion to clumpy, irregular star formation regions and rates. The infalling gas accumulates on the edge of the magnetic fields, supported by magnetic tension. When the mass of the infalling gas exceeds some threshold mass, its gravitational force cannot be balanced by magnetic tension anymore, and it falls toward the disk's plane, rapidly making stars. Simplified estimations of this threshold mass are consistent with clumpy star formation observed in SINS, UDF, GOODS, and GEMS surveys. We discuss the shortcomings of pure hydrodynamic codes in simulating the accretion of cold flows into galaxies, and emphasize the need for magnetohydrodynamic simulations. © 2009. The American Astronomical Society.